Customizing shortcuts in 2.8+ vs 2.79
There are two problems that need to be solved:

The shortcut you're attempting to assign is already in use for the viewport navigation. You may want to change these so you're still able to use them with a different key binding.
The identifier for the operator is not correct, but creating a new shortcut isn't necessary since the mode selection already exists in the default keymap. You only need to adjust the key binding from the regular 1, 2 and 3 keys to the numpad ones.

In order to solve problem 1):

Open the keymap preferences
Switch the filter method to Key-Bindings.
Search for &quot;Numpad&quot;
Scroll down to find the 3D View section and adjust the shortcuts that use Ctrl + Numpad 1, Ctrl + Numpad 2, Ctrl +Numpad 3. (If on a Mac there will also be Cmd + Numpad 1, Cmd + Numpad 2, Cmd +Numpad 3, in which case either the Ctrl or Cmd set can be deactivated, freeing up some shortcuts.)


In order to solve problem 2):

Open the keymap preferences
Switch the filter method to Name
Search for &quot;Select Mode&quot;, the identifier for the operator is mesh.select_mode.
Adjust the key binding to your liking.